In order for a wireless receiver to be able to acquire meaningful data from a transmitter, it is necessary for the receiver to be properly synchronized with the received signal. That is to say, the receiver must sample the data at the correct timing with respect to the received signal. To achieve this, the transmitter must broadcast some kind of synchronization signal whose form is known to the receiver, and which the receiver can use as a reference to determine the timing of the received data.
For example, to acquire a signal in a W-CDMA system, the receiver synchronizes to three broadcast channels, the Primary Synchronization Channel (P-SCH), the Secondary Synchronization Channel (S-SCH) and the Common Pilot Channel (CPICH). For the initial cell search, a primary synchronization is first performed using the P-SCH to provide information on the slot timing, and then a secondary synchronization is performed using the S-SCH to provide information on the code group and frame boundary (i.e. slot position within a frame, or frame boundary acquisition).
One method for performing a synchronization search is discussed in a paper titled “Cell Search Robust to Initial Frequency Offset in WCDMA Systems” (June Moon and Yong-Hwan Lee, School of Electrical Engineering, Seoul National University; from the 13th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 2002; ISBN: 0-7803-7589-0). Summarized briefly, this involves correlating the received P-SCH with a locally generated version of the P-SCH code using a matched filter, then averaging the results over a number of time slots using a coherent combing scheme before performing a peak detection followed by the secondary synchronization.
A difficulty with synchronization is discussed in relation to FIG. 1, which illustrates schematically a wireless communication system 1 comprising a transmitter 2 having a first clock (cktx) 6 and a receiver 4 having a second clock (ckrx) 8. For example, the transmitter may be a base station of a cellular W-CDMA network and the receiver 4 may be a mobile terminal.
The transmitter 2 transmits data to the receiver 4 at a first frequency ftx dependent on the first clock (cktx) 6, including the P-SCH, S-SCH and CPICH. The receiver 4 samples the data at a second frequency frx dependent on the second clock (ckrx) 8. The first frequency ftx and second frequency ftx are nominally the same. However, in reality no two clocks can ever be completely identical, for example due to manufacturing differences. Therefore the second clock (ckrx) 8 will in practice be slightly (but not necessarily negligibly) different from the first clock (cktx) 6, resulting in a frequency offset f0 such that frx actually equals ftx+f0. In the case where the receiver is a mobile terminal such as a mobile phone, the frequency error is likely to be particularly significant because the crystal oscillators used in the clocks 8 of such consumer devices tend to be cheaper and therefore more prone to error and manufacturing spread.
Because of the nature of the received data, any frequency difference f0 between the transmitter frequency ftx and the receiver frequency frx will be seen as an ordinary stationary data vector spinning around the origin, i.e. multiplied by a factor ejφt where φ is dependent on the frequency error f0.
Moon & Lee describe a synchronization method which is more robust against such frequency errors due to an improved, coherent combining scheme. However, one point which is overlooked by the above Moon & Lee reference is that there will be a time shift because of the receiver sampling clock 8 being different from the transmitter clock 6. That is, because data is sampled at a slightly different rate than it is transmitted, the sampling shift will be perceived as a constant movement in time of the data, or a sliding of the received data relative to the transmitted data.
After attempting primary synchronization for example, the secondary synchronization may fail, because the data is not where it is expected to be and so the correlation values returned may be very poor. The degree of averaging and hence the accuracy of the estimation is therefore also determined by the time drift. In practice, to reliably detect a P-SCH at the cell edge limits the allowable frequency error using current techniques is between 1 kHz and 2 kHz depending on implementation. This degree of frequency correction is usually termed fine AFC (automatic frequency control).
Coarse AFC, for correction of frequency errors greater than this, would typically involve trial and error searches with the receiver clock adjusted in incremental amounts for each search. This is a slow process.
It would be advantageous to provide a fast and accurate method of frequency error estimation. It would also be advantageous to be able to cope with larger frequency errors. Particularly, in a case where the receiver 4 is a mobile consumer device such as a mobile phone, it would be preferable to be able to use a cheaper oscillator for the clock 8.